The present invention relates to a connector for an NOx sensor equipped with input and output terminals for inputting a signal to and outputting a signal from an NOx sensor that measures an NOx concentration.
NOx sensors for measuring the concentration of nitrogen oxides (NOx) contained in gas to be measured (hereinafter referred to as a measurement gas) are disclosed in, for example, EP-A1-0678740 and SAE Paper No. 960334, pp. 137-142, 1996. Such a conventional NOx sensor is composed of oxygen-ion conductive solid electrolyte layers that form a first measurement chamber and a second measurement chamber. The first measurement chamber communicates with a measurement gas via a first diffusion-controlling layer, and the second measurement chamber communicates with the first measurement chamber via a second diffusion-controlling layer. A first oxygen-pumping cell and an oxygen-concentration-measuring cell are formed adjacent to the first measurement chamber and are each composed of a solid electrolyte layer sandwiched between porous electrodes. A second oxygen-pumping cell is formed adjacent to the second measurement chamber and is composed of a solid electrolyte layer sandwiched between porous electrodes.
In order to obtain the concentration of NOx contained in a measurement gas, such as exhaust from an internal combustion engine, without influence of other gas components (such as oxygen, carbon monoxide, and carbon dioxide) contained in the measurement gas, the above Nox sensor undergoes the following control.
Electrical current (hereinafter referred to as first pump current) is caused to flow to the first oxygen-pumping cell such that an output voltage from the oxygen-concentration-measuring cell attains a predetermined value, thereby controlling the concentration of oxygen contained in the first measurement chamber to a very low level (for example, about 1000 ppm). The measurement gas whose oxygen concentration has been thus controlled to the low level enters the second measurement chamber. A constant voltage is applied to the second oxygen-pumping cell in such a direction that oxygen is pumped out from the second measurement chamber. Then, by virtue of a catalytic function of the porous electrodes of the second oxygen-pumping cell, the second oxygen-pumping cell decomposes NOx contained in the measurement gas into nitrogen and oxygen and pumps out the thus-obtained oxygen from the second measurement chamber. At this time, on the basis of current flowing through the second oxygen-pumping cell, the NOx concentration of the measurement gas is obtained.
The first pump current is controlled such that the concentration of oxygen contained in the first measurement chamber attains low level, but not zero, for the following reason. If the concentration of oxygen contained in the first measurement chamber is controlled to zero, the porous electrodes that constitute the first oxygen-pumping cell cause decomposition of NOx contained in the measurement gas that has entered the first measurement chamber. This disables measurement of NOx concentration through use of the second oxygen-pumping cell.
The above NOx sensor can obtain not only the concentration of NOX contained in the measurement gas from the second pump current but also the concentration of oxygen contained in the measurement gas from the first pump current.
However, a characteristic indicative of the relationship between NOx concentration and second pump current and that between oxygen concentration and first pump current somewhat differ among NOx sensors. If, on the assumption that NOx sensors have certain common characteristics indicative of the relationships, the NOx concentration and the oxygen concentration of a measurement gas are obtained from the second and first pump currents, respectively, detected by the NOx sensor, some NOx sensors may fail to provide sufficiently high measurement accuracy.
An object of the present invention is to provide sufficiently high accuracy in measurement by an NOx sensor regardless of variations in characteristics among NOx sensors.
According to the present invention there is provided a connector for an NOx sensor, which connector comprises input and output terminals for inputting a signal to and outputting a signal from the NOx sensor. The NOx sensor comprises a first measurement chamber and a second measurement chamber. The first measurement chamber comprises a first oxygen-pumping cell and an oxygen-concentration-measuring cell and communicates with a measurement gas via a first diffusion-controlling layer. The first oxygen-pumping cell and the oxygen-concentration-measuring cell are each formed of an oxygen-ion conductive solid electrolyte layer sandwiched between porous electrodes. The second measurement chamber comprises a second oxygen-pumping cell and communicates with the first measurement chamber via a second diffusion-controlling layer. The second oxygen-pumping cell is formed of an oxygen-ion conductive solid electrolyte layer sandwiched between porous electrodes. A first pump current is caused to flow to the first oxygen-pumping cell such that an output voltage from the oxygen-concentration-measuring cell is maintained at a constant value so as to control the concentration of oxygen contained in the first measurement chamber to a constant level. A constant voltage is applied to the second oxygen-pumping cell in such a direction that oxygen is pumped out from the second measurement chamber. There is detected a second pump current that flows through the second oxygen-pumping cell as a result of the application of the constant voltage and according to the concentration of NOx contained in a measurement gas.
The connector for an NOx sensor is equipped with a label resistor having resistance corresponding to at least either a relationship between the concentration of oxygen contained in the measurement gas and the first pump current or a relationship between the concentration of NOx contained in the measurement gas and the second pump current, and is equipped with a pair of label signal output terminals connected to opposite ends of the label resistor.
The thus-configured connector for an NOx sensor is used in the following manner. The resistance of the label resistor is detected via the label signal output terminals to thereby identify characteristics of the NOx sensor concerned. Subsequently, the first pump current is caused to flow to the first oxygen-pumping cell such that an output voltage from the oxygen-concentration-measuring cell becomes constant, i.e., such that the concentration of oxygen contained in the first measurement chamber becomes constant. Also, a constant voltage is applied to the second oxygen-pumping cell in such a direction that oxygen is pumped out from the second measurement chamber. The NOx concentration and the oxygen concentration of the measurement gas are respectively obtained from the second pump current, which flows through the second oxygen-pumping cell according to the concentration of NOx contained in the measurement gas, and the first pump current, which flows according to the concentration of oxygen contained in the measurement gas, as well as on the basis of a characteristic(s) of the NOx sensor identified by the resistance of the label resistor.
Accordingly, through use of the connector of the present invention for an NOx sensor, a characteristic (at least either relationship between NOx concentration and second pump current or relationship between oxygen concentration and first pump current) of an NOx sensor can be identified by the resistance of the label resistor. Through employment of the identified characteristic(s) of the NOx sensor, the NOx concentration and the oxygen concentration of the measurement gas can be obtained at a sufficiently high degree of accuracy.
Through identification of the relationship between NOx concentration and second pump current, variations in an obtained value of NOx concentration among NOx sensors can be compensated for. Through identification of the relationship between oxygen concentration and first pump current, the concentration of oxygen contained in the measurement gas can be accurately obtained; thus, through compensation of the second pump current on the basis of the thus accurately obtained oxygen concentration, the NOx concentration of the measurement gas can be accurately obtained against variations in the concentration of oxygen contained in the measurement gas.
Further, through identification of both of the above relationships implemented by a minor additional element of a label resistor, the measurement accuracy of the NOx sensor can be significantly improved.
The resistance of the label resistor may correspond to at least the sensitivity of the second pump current to variations in the concentration of NOx contained in the measurement gas.
Problems Solved by the Invention
In the accompanying drawings FIG. 6B schematically shows the relationship between the second pump current and the concentration of NOx contained in the measurement gas. Offset of the second pump current (second pump current at an NOx concentration of 0%) and sensitivity (slope of the graph) differ among NOx sensors.
Through use of a relative value, as opposed to an absolute value, for the second pump current in execution of control, offsets are canceled; thus, through mere compensation for variations in sensitivity, measurement can be performed accurately.
The resistance of the label resistor may correspond to at least the first pump current as measured when the oxygen concentration of the measurement gas is identical to that of the atmosphere.
Again, FIG. 6A schematically shows the relationship between the first pump current and the concentration of oxygen contained in the measurement gas. As shown in FIG. 6A, the first pump current has an offset of 0 and is substantially proportional to the oxygen concentration. Thus, when the first pump current at a certain oxygen concentration is known, the sensitivity of the first pump current to the oxygen concentration can be easily obtained.
Accordingly, from a first pump current as measured when the measurement gas assumes an oxygen concentration identical to that of the atmosphere, the relationship between the first pump current and the concentration of oxygen contained in the measurement gas can be identified. On the basis of the thus-identified characteristic relationship, the concentration of oxygen contained in the measurement gas can be accurately obtained.
Through use of the thus-obtained oxygen concentration, the oxygen concentration dependency of an NOx sensor can be compensated for. Also, the thus-obtained oxygen concentration can be applied to various kinds of control that are performed on the basis of the oxygen concentration of the measurement gas.
The resistance of the label resistor may correspond to not only oxygen concentration but also to air-fuel ratio or excess air factor.
The invention will be further described by way of example with reference to the accompanying drawings, in which: